1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory having an electric programmable/ erasable function.
2. Description of the Related Art
A memory referred to as an "electric one-time erasable NOR type flash memory" has been developed in the past as a nonvolatile semiconductor memory as described, for example, in JP-A-62-27687 (laid open on Dec. 1, 1987) and JP-A-3-219496 (laid open on Sep. 26, 1991).
FIG. 7 of the accompanying drawings illustrates a schematic sectional structure of the NOR type flash memory cell according to the prior art and its operation. The NOR type flash memory cell according to the prior art comprises a floating gate type field effect transistor structure wherein a gate oxide film 2, a floating gate 3, an intermediate insulating film 4 and a control gate 5 are formed on a p type silicon substrate 1, and n type impurity layer 22 is formed on a source terminal side and an n type impurity layer 23 and a p type impurity layer 24 are formed on a drain terminal side.
The NOR type flash memory according to the prior art is formed by arranging the memory cells described above in matrix, connecting the drain terminal of each memory cell to a data line, connecting each source terminal to a common source line, and connecting each control gate to a word line.
Memory cell data is erased by applying a negative voltage to the control gate 5 and a positive voltage to the source impurity layer 22. At this time, a high electric field is applied to the gate oxide film 2 and a tunneling mechanism of electrons takes place, so that electrons accumulated in the floating gate 3 are pulled out to the source impurity layer 22. A threshold voltage of the memory cell decreases due to this erasing operation.
Programming of data into the memory cell is effected by applying a positive voltage to the drain impurity layer 23 and to the control gate 5. At this time, hot electrons generated in the vicinity of the surface of a drain junction are injected into the floating gate 3. A threshold voltage of the memory cell increases due to this programming.
The NOR type flash memory according to the prior art described above have the function of collectively erasing at one time a chip as a whole or a certain groups of memory cells, and one transistor can constitute one memory cell. Further, when a circuit scheme wherein a source wiring is used in common for all bits, is employed, the memory chip area can be reduced.
In comparison with the NOR type flash memory cell according to the prior art described above, a nonvolatile semiconductor memory is known which utilizes a Fowler-Nordheim (F-N) tunneling mechanism.
An ACEE (Advanced Contactless EEPROM) described in IEEE Journal of Solid-State Circuits, Vol. 4, No. 4, April, 1991, pp. 484-491, is one of the examples of the non-volatile semiconductor memory described above. Transistors used for this ACEE are those transistors which have a thin oxide film region for the F-N tunneling at only an overlapped portion between the floating gate and the source, and the thickness of the oxide film of the transistor region is set to be greater than the thickness of the oxide film in the tunnel region. The memory cells are arranged in matrix, the drain terminal of each memory cell is connected to a data line comprising an impurity layer, and the source terminal is connected to a source line comprising mutually different impurity layers. Further, the impurity layer data line and the impurity layer source lines connected to a plurality of memory cells are connected to a data line and to a common source line through a MOS transistor (select transistor), respectively.
The device operations are as follows. In the erasing operation, a negative voltage (-11 V) is applied to a selected control gate to turn ON a source side select transistor and a positive voltage (5 V) is applied to the common source terminal, so that electrons are released from the floating gate through the tunnel region on the source side of the selected memory cell. In the programming operation, the drain side select transistor is turned ON with the source side select transistor being kept OFF, a positive voltage (18 V) is applied to the selected control gate, a positive voltage (7 V) is applied to the non-selected control gate to such an extent that programming is not made, 0 V is applied to the data line so as to set the voltage on the source side to 0 V through the non-selected memory cells which commonly share the data line but to which programming is not made, and the electrons are thus injected into the floating gate from the source side tunnel region of the selected memory cell by utilizing the F-N tunneling mechanism. Here a 7 V voltage is applied to the data line for those memory cells which share in common the control gate voltage with the memory cell to be subjected to programming but into which programming is not made, and the electric field applied to the source side tunnel region is relaxed.
Since the ACEE utilizes the F-N tunneling mechanism for the programming/erasing operations, a consumed current per bit is small and hence, a voltage booster having small current drivability can be used inside the chip. Accordingly, a single 5 V supply can be used.
A nonvolatile semiconductor memory utilizing the F-N tunneling mechanism is also described in JP-A-4-14871 (laid open on Jan. 20, 1992). This nonvolatile semiconductor memory uses a floating gate type field effect transistor structure for memory cells, and has the structure wherein the drains of a predetermined number of memory cells are connected to a main bit line, this sub bit line is connected to a main bit line through a MOS transistor, and the source terminals are connected in common to the source line.
To erase memory cell data, a positive voltage V.sub.F (e.g. 22 V) is applied to the control gate, and the source terminals and the drain terminals are first grounded so as to accumulate the electrons in the floating gate. In the programming operation, the control gate of a selected memory cell is grounded and the positive voltage V.sub.F is applied to the drain impurity layer. To inhibit programming, a voltage V.sub.F /2 is applied to the drain terminals. Accordingly, the electrons are released from the floating gate to the drain impurity layer in the select memory cell due to the tunneling mechanism.
The non-volatile semiconductor memory using the F-N tunneling mechanism effects the programming/erasing operations of data by the use of a very small current, that is, the tunnel current. Accordingly, this semiconductor memory is effective for accomplishing lower power consumption.
An EEPROM described in IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 5, October 1982, pp. 821-827, is another example of the nonvolatile semiconductor memory using the F-N tunneling mechanism. In this EEPROM, the electrons are injected from the drain to the floating gate and attain a low threshold voltage in the programming operation, and the electrons are released from the floating gate to the whole channel immediately therebelow and attain a high threshold value. The cell of this EEPROM comprises a floating gate type F-N tunnel transistor and a selector transistor connected to the drain side of the former. The memory cells are arranged in matrix, the drain terminal of the select transistor of the memory cell is connected to the data line through a switch transistor outside the memory cell, and the source terminal of the floating gate type F-N tunnel transistor of the memory cell is directly connected to the common source line.